This invention relates to a valve gated injection molding system and more particularly to a valve pin bushing for use in such a system.
In a multi-cavity molding system in which the melt passage branches outward through a manifold plate, it is well known to provide a heated nozzle or sprue bushing seated in the cavity plate in alignment with each of the gates. As may be seen in the applicant's U.S. patent application Ser. No. 360,828 filed Mar. 23, 1982 entitled "Hydraulically Actuated Injection Molding System", the valve pin extends through the heated nozzle from the actuating mechanism to the gate. The hot manifold plate extends between the cooled back plate supporting the actuating mechanism and the cooled cavity plate in which the heated nozzle is seated. Therefore, in order to avoid alignment problems between the valve pin, the actuating mechanism and the gate due to lateral thermal expansion of the manifold plate, it is necessary that ample clearance be provided between the valve pin and the manifold plate.
The melt passage from the manifold plate joins an enlarged portion of the valve pin bore in the heated nozzle and the valve pin fits snugly in the remaining smaller portion of the bore. In order to avoid leakage of the pressurized melt as the valve pin reciprocates, it has been found that this smaller diameter portion must have a minimum length relative to the diameter of the valve pin. In the system shown in above-mentioned U.S. application Ser. No. 360,828, this structure is provided in a bushing seal which is seated in the nozzle as described in the applicant's U.S. Pat. No. 4,026,518 entitled "Bushing Seal for Valve Gated Injection Mold" which issued May 31, 1977. However, it is desirable that a system be provided which utilizes heated nozzles or sprue bushings of the type described in the applicant's U.S. patent application Ser. No. 285,260 filed July 20, 1981 entitled "Sprue Bushing Connector Assembly and Method" but, as described above, it is undesirable that the seal around the valve pin be provided by having it in contact with the manifold plate.
Furthermore, it will be appreciated that in the system described in above-mentioned U.S. application Ser. No. 285,260, when the manifold plate expands and contracts, it exerts lateral frictional forces on the heated nozzle due to the considerable bearing contact between them. This has the undesirable effect of tending to tip or cock the heated nozzle on its seat in the cavity plate which interferes with the critical alignment of the valve pin and can result in leakage of the melt.